A technique has been developed that permits the isolation of pure populations or heterokaryons between cells of any type without the need for selective genetic markers. It will be used to investigate the control of gene expression during cell differentiation and aging. Teratocarcinoma, dividing skeletal myoblastic, postmitotic skeletal myogenic, and cardiac cells from various species will be fused in appropriate combinations to reveal patterns of control within single developmental or phenotypically related lineages. The new approach of "common-partner heterokaryon analysis" will be introduced to unmask changes in regulatory molecules between young and old cells, and to distinguish between suppressor versus dilution-of-an-activor models for the suppression of functions in hybrid cells. Results will be analyzed using two-dimensional gel electrophoresis, immunofluorescence and autoradiography. Nuclear versus cytoplasmic control of observed phenotypic interactions will be explored by studying pure populations of heteroplasmons between anucleate cytoplasms and whole cells. The results of these experiments should not only provide substantial insights into the mechanisms controlling cell differentiation and aging, but should demonstrate the general utility of this approach for studying the control of gene expression in vertebrate cells.